Preparation of fiber-reinforced metal alloy composites by compaction in the semimolten phase



A. P DIVECHA ETAL 3,441,392 PREPARATION OF FIBER-REINFORCED METAL ALLOY COMPOSITES BY April 29, 1969 COMPACTION IN THE SEMIMOLTEN PHASE Filed March 27, 1967 m/////// A7 4V OOO OO%@ 3 0A m mH N 5 m V N w M,/Zar

United States Patent Oflice Patented Apr. 29, 1969 3,441,392 PREPARATION OF FIBER-REINFORCED METAL ALLOY COMPOSITES BY COMPACTION IN THE SEMIMOLTEN PHASE Amarnath P. Divecha, Falls Church, and Henry Hahn, Fairfax, Va., and Paul J. Lare, Bowie, Md., assignors to Melpar, Inc., Falls Church, Va., a corporation of Delaware Filed Mar. 27, 1967, Ser. No. 626,190 Int. Cl. 1322f 3/14 US. Cl. 29182.8 26 Claims ABSTRACT OF THE DISCLOSURE Processes of preparing metal alloy-whisker composites exhibiting significant increases in mechanical strength over alloys without fiber reinforcement, by hot pressing the composite with the metal matrix in a partly solid and partly liquid state.

BACKGROUND OF THE INVENTlION The present invention relates generally to reinforcement of metals and metal alloys, and in particular to processes for incorporating high strength fibers in a desirable orientation or alignment in the metal matrix to provide reinforced metal-whisker composites.

The advent of high strength, single-crystal fibers (whiskers), and the availability of sapphire and silicon carbide whiskers in ever-increasing quantities have generated extensive interest, study and experimentation in the incorporation of whiskers in metallic matrices, so as to combine the mechanical properties of ductile metals with those of high strength fibers, oriented at will to manifest reinforcement in the direction of the applied stresses.

While fiber reinforcement of metals has been proposed and investigated in the past, a significant problem has been encountered in the attainment of specimens of reinforced composites in sizes sufficiently large for practical use and for meaningful evaluation of working characteristics.

A basic requirement of a successfully reinforced composite lies in its ability to transfer load from one whisker to another.

Another requirement is the attainment of suitable whisker-matrix interface bonding. Wetting of the fibers by the matrix is also necessary, and it has been found for example that pure molten nickel bonds to alumina r fibers under prolonged contact (about 30 minutes), and that very small additions of chromium (approximately 1%) also enhances wetting.

In the past, fiber-reinforced metal composites have generally been prepared by liquid infiltration and solid state powder metallurgy techniques, yielding very small composites and demonstrating little potential in terms of reliability or upscaling. In the liquid infiltration process, metallized whisker mats are immersed or otherwise brought into contact with the molten metal, frequently resulting in dissolution of the whisker coating before complete encapsulation of the whiskers by the metal can occur. Accordingly, incomplete bonding may exist between fibers and metal matrix of the composite, as well as porosity of the final structure.

Variations of the standard power metallurgical techniques by which to achieve consolidation of whiskermetal matrix composites have included cold or hot pressing of whisker-metal powder mixtures, followed by sinter- Another method involving pressing requires the use of metallized whisker mats, yarns, or other suitable shapes,

which are electroplated and hot pressed into fully dense structures. These prior art methods which employ pressing, however, produce significant whisker damage during compaction at high. pressures. Low pressure compaction on the other hand results in porous structures which require sintering over long periods of time. In any case, the previous pressing techniques have failed to produce complete encapsulation of the individual fibers by the metal.

Still another prior art method of producing fiber-reinforced composites comprises a powder metallurgy technique in which the metal or alloy to be reinforced is comminuted to a fine powder and mixed with the fibers, a plasticizer, and an alkaline earth metal, at room temperature. The mixture is thereafter kneaded to a plastic mass at room temperature, extruded, dried, sintered, cooled, and resintered, the sintering steps accomplished in a flowing hydrogen atmosphere. \In addition to the number of operations required, only very small specimens of composites have thus been produced, although of relatively high tensile strength and with substantially predetermined fiber orientation.

A prior art fusion technique of preparing the whiskerreinforced composites includes the same initial mixing step as employed in the powder metallurgy technique described immediately above, except that the plasticizer is omitted. The mixture is dried at elevated temperature, comminuted, and heated to its fusion temperature for a short period. Although fewer steps are required than in the immediately preceding technique, this process again results in small composite specimens, along with some loss of integrity in the final structure as a result of an oxidation reaction.

SUMMARY OF THE INVENTION Accordingly, it is a broad object of the present invention to provide improved processes for preparing fiberreinforced composites.

It is a more specific object of the present invention to provide methods of fabricating whisker-reinforced composites under pressure with the metal matrix material in a semimolten state, by which near theoretical density in the composite is achieved.

Another object is to provide methods of producing whisker-metal alloy composites in which substantially complete encapsulation and wetting of the whiskers are achieved during consolidation.

Still another object is to provide methods according to the above-stated objects, which methods are capable of producing small scale whisker-metal alloy composites or are readily upscaled to permit reliable preparation of relatively large-sized composites, using existing apparatus.

Briefly, in accordance with an exemplary process illustrating the concepts and principles of the present invention, the above objects and advantages are achieved by performing the compaction in the semimolten region of the metal matrix material. More specifically, the matrix material in the form of powder is mixed with the coated or uncoated fibers, in loose separated form, and the mixture placed in the die of a conventional hot pressing apparatus where it is heated to a temperature silghtly below the solidus of the system, at which point a predetermined constant pressure is applied to the system and the temperature of the composite raised to cross over into the semimolten region of the matrix. Pressure is maintained until desired compaction is achieved, the matrix completely encapsulating and bonding to the fibers, after which the composite is cooled and removed, and hot rolled to produce a preferred fiber orientation.

BRIEF DESCRIPTION OF THE DRAWING The above and still further objects, features and attendant advantages of the present invention will become apparent from a consideration of the sole figure, in which there is shown a schematic sectional view of a typical hot pressing apparatus suitable for use in the process of the invention.

DESCRIPTION OF THE PREFERRED PROCESS In the processes to be described, reference is made to certain specific materials as suitable for use as fibers (whiskers), and others as suitable for the metal matrices. It is to be emphasized that these are recited as purely illustrative of the large number of fiber materials and metal matrix materials conventionally employed in the preparation of fiber-reinforced composites.

In general, the fibers employed in the process ranged up to approximately 30 microns in diameter and up to about one-half inch in length, of alpha alumina, such as sapphride, or of silicon carbide (a-SiC). Either uncoated or coated (e.g., sapphire electroplated with nickel) whiskers, or both, may be used in the fabrication of a composite. Suitable fibers are available, for example, from Thermokinetic Fibers, Inc., Nutley, N.J., or may be produced locally if appropriate facilities exist.

In the preparation of each fiber reinfroced metal composite by compaction in the semimolten region of the metal matrix, the selected matrices are preferably alloys rather than pure metals, since liquid-phase hot pressing requires a system with distinct two-phase (e.g., liq.+a) region. While mechanical mixtures as well as prealloyed powders of the individual components of the matrix may be utilized, the prealloyed powder is preferable, having the advantage of a known distinct melting point. Additionally, there is less likelihood of production of an inhomogeneous structure resulting from the formation of an undesirable intermetallic phase during heating of the compact.

Among the mixed and prealloyed powders used in the preparation of composites according to the procedures to be described were Al-Si 325 mesh prealloyed atomized powder composed of 10.2 Si, 0.03 Mg, 0.16 Fe, balance Al (percent by weight); Al-Cu 325 mesh prealloyed atomized powder composed of 4.5 Cu, balance Al; and 7075 A1 325 mesh prealloyed atomized powder composed of 0.3 Si, 0.6 Fe, 2.1 Cu, 0.2 Mn, 2.2 Mg, 0.2 Cr, 0.01 Ni, 4.7 Zn, balance Al, all of these prealloyed powders available from Reynolds Aluminum Company, Richmond, Va.; Nichrome 80 Ni20 Cr 325 mesh prealloyed atomized powder; and silicon (+98 Si, balance impurities) 325 mesh elemental silicon, both available from Consolidated Astronautics, New York; nickel (+99.7 Ni, balance impurities) 4-7 microns, prepared by carbonyl process, available from International Nickel Company, New York; and cast nickel 325 mesh prealloyed atomized powder, composed of 1 Si, 1 Mn, balance Ni, available from Hoegannes Sponge Iron Co., New Jersey To illustrate a process according to the present invention, the following nonlimiting example of the preparation of a sapphire fiber reinforced Al-Si composite is presented.

The A1 fibers are dispersed in isopropyl alcohol to assure freedom from clumps and clusters existing in the initial form, by agitation in a suitable blender or stirring unit at medium speeds. The amount of time and the speed at which adequate dispersion is obtained varies with the type and size of the whiskers. For example, initially loose sapphire fibers are dispersed by agitation in 250 cc. of isopropanol per gram of fibers (cc/gm.) at a blending speed setting of 3 or 4 (a household Osterizer blender speed transposable to revolutions per minute of the stirring blades) for a period of approximately three minutes. As a general proposition, smaller diameter fibers require longer periods of time for uniform dispersion. Grown mats or wool clusters of sapphire whiskers require severe agitation to break up the clusters. Metallized sapphire whisker mats, for example, are mixed with 250 cc. isopropanol per gm. and agitated at medium speed (Osterizer setting 34) for about 10 minutes. The same is true for dispersion of u-SiC whiskers. Care must be exercised to minimize damage to the fibers, especially in the case of metallized whiskers where it has been found that steel blades in the agitator can damage or even remove the coating. In such instances, soft polymer stirrers should replace the blades.

After the whiskers have been dispersed, -325 mesh All0.2 Si (percent by weight) metal powder is added to the whisker-isopropanol slurry in such amount as to produce a fiber-metal composite of Al+10.2 (percent by weight) Si+l5 (percent by volume) A1 0 (l30,u) whiskers and the blending or stirring resumed to thoroughly mix the ingredients. An additional mixing duration (beyond that to produce initial dispersion of the fibers) of approximately two minutes has been found suflicient after addition of substantially any of the metal powders to substantially any fiber slurry. While the suspension is in motion, the mixture is filtered by aspiration through #31 Whatman paper. The Whisker-metal blend may then readily be examined under a microscope for uniformity. It has been found at this point, upon examination at about 30x magnification, that the whiskers are randomly oriented in the plane of the filter paper.

The collected whisker-metal mixture, after drying, is compacted in a suitable hot pressing apparatus such as that schematically shown in the figure. The specimen 10 (whisker-metal mixture) is placed in a die 12 (1% inches) on a plug 15 and is heated by induction via coil 17.

Low melting alloys such as aluminum are prepared using a tool steel die, while matrices such as nickel-silicon and Nichrome are preferably prepared using a graphite die. Even in the case of low melting alloys, it is desirable to utilize both plunger 20 and plug 15 of graphite to prevent seizing. Moreover, for a steel die the walls are coated with suitable lubricant, such as molybdenum disulfide, to enable rapid ejection of the specimen after hot pressing.

Die 12 is encompassed by graphite cloth 23 and encased in fused silica tube 25, a typical configuration for a convntional laboratory hydraulic press. The remaining structure of the press includes a base 28 of dense refractory brick, an alumina block 30 interposed between the graphite plunger 20 and the hydraulic ram 35, and a thermocouple 37 extending into an aperture in die 12, the temperature sensing element of the couple being in secure heat conductive contact with the die.

Whisker-metal mixture 10 is heated slowly to a temperature slightly below the solidus of the matrix system. At that point, a pressure of 2000 p.s.i. is applied and the temperature is further raised to cross over into the (liquid-i-a) semimolten range. In the particular example of matrix and whisker materials and percentages presently under discussion, in which Al10.2 w/o Si-15 v/o A1 0 (i.e., 10.2% by weight of silicon, 15% by volume of alumina whiskers) composite is prepared, the latter temperature is from approximately 575 to 585 C. A precise indication of the achievement of this temperature is its accompaniment by a sudden drop in pressure at the point of crossover into the (liquid+a) semimolten range. Pressure is returned to 2000 p.s.i. and maintained there during the hot pressing operations. Thermocouple 37 is used merely as a reference to determine the true solidus temperature. For aluminum and aluminum base alloys, the compaction is performed in air, whereas for nickel base alloys an inert atmosphere (e.g., N or A) or partial ly reducing atmosphere (e.g., N +10 is utilized.

Extreme care should be exercised to insure that the hot pressing temperature does not rise above the liquidus, otherwise excessive amount of metal will be squeezed out and carry with it a large percentage of the fibers.

After the liquid-phase hot pressing operation is carried out, the die assembly is allowed to cool, is disassembled, and the compacted composite is removed. Prior to mechanical working, the composite density may be measured by alcohol or mercury immersion, and has been found typically to range upward of 97 percent of theoretical density. We have observed that cold rolling of the composite causes considerable fiber damage even in a low modulus matrix such as aluminum; hot rolling, on the other hand, tended to align the fibers as well as to minimize fiber fracture. In general, hot rolling temperatures of from 370 to 390 C. were utilized for Al-Si matrices, and as in the case of other composites, all hot working temperatures are in accord with recommended data for the particular system. Roll reduction was approximately 80% or less.

Tensile strength of specimens (generally coupons) was measured on an Instron tensile testing machine. Elevated temperature testing was performed by use of a resistance furnace, and all testing was performed in air. Elastic modulus was measured on a Tinius Olsen machine equipped with an extensometer.

For composites of aluminum-silicon metal matrix containing from 5 to 40 volume percent (preferably to volume percent) of alumina whiskers, significant reinforcement was achieved at room and elevated temperatures over the strength of the metal without fibers. In particular, the aluminum-silicon composites exhibited an average increase of 60 percent in tensile strength with attendant average increase of 50 percent in elastic modulus, at room temperature. This strengthening was achieved with both coated and uncoated fibers. Greater reinforcement was observed at elevated temperatures; for example, at 800 F. the alloy composite has a tensile strength' greater than commercial aluminum (wrought alloy) by a factor of from two to three. Composites of other aluminum-base matrices, such as 7075 Al-Cu, reinforced with from about 5 to about 40 volume percent fibers showed less improvement. For the latter composites, liquid-phase hot pressing temperatures ranged upward to 500 C., but this again depends upon the temperature at which crossover to the (liq.+a) semimolten region of the composite occurs.

The same basic procedure as that described above is utilized for the nickel-base alloy composites and other matrix composites, with appropriate changes in process parameters such as temperature. Protective and reducing atmospheres are employed where necessary. For example, compositions of Ni-2 (by weight) Ag with 5 to 40 volume percent alumina or silicon carbide fibers are hot pressed at approximately 970 and hot rolled at about 775 C. with roll reduction of 70 percent or less. Substantially pure nickel or copper metal matrices as well as other nickel-base alloy (e.g., Nichrome and Ni8Si) matrices with similar percentages by volume of fibers to those described above also provide fiber-reinforced composites, but of generally less improvement in mechanical properties than that exhibited by the aluminum-base alloys.

We claim:

1. Process for the preparation of fiber-reinforced metal alloy composites comprising forming a mixture of (a) a powdered metal alloy having a distinct stable semimolten region and including at least one member selected from the group consisting of aluminum, nickel, and copper and at least one other member selected from the group consisting of silicon, silver, magnesium, chromium, and zinc, and (b) ceramic fibers in the approximate percentage by volume of from 5 to 40% of the total mixture, heating said mixture to a temperature below the solidus of the metal alloy matrix, applying a predetermined pressure to said mixture sufiicient to produce encapsulation of the fibers by the matrix when the matrix is in its semimolten region, increasing the temperature of said matrix to the semimolten region thereof, maintaining said predetermined pressure on said mixture until the desired compaction is achieved, and cooling the composite thus formed.

2. The process of claim 1 further including the step of hot rolling the composite formed, to orient the fibers therein in a preferred direction.

3. The process of claim 1 wherein said alloy is composed essentially of aluminum and silicon in the approxi mate percentage by weight of 10:1, respectively.

4. The process of claim 3 wherein said fibers are alumina.

5. The process of claim 4 wherein said alumina fibers are in the amount of 10 to 15 percent by volume of the total composite.

6. Process of preparing metal alloy-fiber composites which comprises combining ceramic fibers with the desired metal alloy matrix, hot pressing the composite so formed at a predetermined pressure with the composite heated to a temperature in which the metal matrix is ill the solid phase, increasing the temperature of said composite to a point in which the metal matrix is in the partly solid, partly liquid state, maintaining said predetermined pressure to produce consolidation and densification of the composite, and cooling the consolidated composite.

7. Process according to claim 6 wherein the fiber content of said composite is in the range from 5 percent to 40 percent by volume.

8. Process according to claim 7 wherein said fiber content is from 10 to 15 percent by volume.

9. Process according to claim 7 wherein said metal alloy matrix is a prealloyed powder consisting essentially of a metal selected from the group consisting of aluminum, nickel, copper, and silicon, together with one or more metals from the group consisting of silver, magnesium, chromium, and zinc.

10. Process according to claim 9 wherein said fibers are selected from the group consisting of alpha alumina and silicon carbide.

11. Process according to claim 10 wherein said fibers are clad with an appropriate metal to enhance wettability by said matrix when said matrix is in the partly solid, partly liquid state.

12. The fiber-reinforced composite produced by the process which comprises mixing a metal alloy powder with high strength ceramic fibers dispersed in an inert liquid vehicle, said alloy exhibiting a distinct stable two phase solid-liquid region, said ceramic fibers being of any specific type that are not degraded by the metal alloy and are sufliciently wetted by the metal alloy when in said two phase region, drying the mixture and then heating the dried mixture to a temperature slightly less than that at which the metal alloy system is in the solidus region, and consolidating said mixture under pressure while rais ing the temperature thereof to the semimolten region of the metal alloy system, but below the liquidus temperature.

13. The composite according to claim 12 wherein said compacted mixture is hot rolled to produce substantial fiber orientation in a preferred direction.

14. The composite according to claim 12 wherein said mixture contains from 5 to 40 percent by volume of said fibers.

15. The fiber-reinforced composite produced by the process which comprises combining a powdered metal alloy matrix with dispersed ceramic fibers and forming an essentially dry mixture thereof, hot pressing the mixture while maintaining it in a semimolten state below the liquids of the system, and cooling the composite when the desired compaction has been achieved.

16. The product according to claim 15 wherein the composite contains from 5 to 40 percent by volume of said fibers.

17. The product formed by the process which comprises combining a powdered metal alloy matrix wtih dispersed reinforcing fibers to form a dry mixture thereof with up to about 40 percent by volume of fibers, heating the mixture to a temperature slightly less than the solidus of the metal alloy matrix, and applying pressure to the heated mixture while raising the temperature thereof to the semimolten region of the metal alloy matrix until the desired compaction is achieved.

18. The product according to claim 17 wherein the compacted mixture is subsequently hot rolled to obtain a preferred fiber orientation.

19. The product according to claim 17 wherein the metal matrix is an alloy having an aluminum, nickel, or copper base.

20. The process of claim 1 wherein said fibers are alumina.

21. The process of claim 1 wherein said fibers are silicon carbide.

22. The process of claim 1 wherein said fibers have diameters ranging upward to approximately 30 microns, and lengths ranging upward to about one-half inch.

23. Process according to claim 10 wherein said fibers range up to about one-half inch in length and up to about 30 microns in thickness.

24. Process of preparing fiber reinforced metal alloy composites, comprising mixing a powdered metal alloy matrix that exhibits a stable two phase semimolten region, with from 5 to 40% by volume of fibers composed of a material selected from the group consisting of alpha alumina and silicon carbide; heating the mixture to a temperature below the solidus of the matrix and applying pressure to the mixture of suflicient magnitude to produce consolidation thereof and to produce infiltration of the semimolten matrix between the fibers when said matrix is. in the semimolten region; increasing the temperature of said matrix to place it in the semimolten region and References Cited UNITED STATES PATENTS 2,137,201 11/1938 Boyer -226 2,953,849 9/1960 Morgan.

3,167,427 1/1965 Slayter 75--206 X 3,282,658 11/1966 Wainer.

3,364,975 1/1968 Gruber 106-44 X OTHER REFERENCES Metals Handbook, 1948 edition, American Society for Metals, Cleveland, Ohio, pages 1062, 1198.

Fiber Composite Materials, American Society for Metals, Metals Park, Ohio, March 1965, pages 206, 207.

CARL G. QUARFORTH, Primary Examiner.

ARTHUR I STEINER, Assistant Examiner.

US. Cl. X.R. 

